Negative electrode active material for rechargeable lithium battery, method for preparing the same, and negative electrode and rechargeable lithium battery including same

ABSTRACT

A negative active material for a rechargeable lithium battery includes a carbon-based active material including highly crystalline natural graphite and artificial graphite. The carbon-based active material has a peak intensity ratio (P2/P4) of about 0.3 to about 0.4, wherein P2 refers to the 101 peak of a rhombohedral crystal grain and P4 refers to the 101 peak of a hexagonal crystal grain, as measured by X-ray diffraction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/226,626, filed Aug. 2, 2016, which claims priority to and the benefitof Korean Patent Application No. 10-2015-0121121, filed on Aug. 27, 2015in the Korean Intellectual Property Office, the entire content of whichis incorporated herein by reference.

BACKGROUND 1. Field

One or more aspects of embodiments of the present disclosure relate to anegative active material for a rechargeable lithium battery, a method ofpreparing the same, a negative electrode including the same, and arechargeable lithium battery including the same.

2. Description of the Related Art

Rechargeable lithium batteries have recently drawn attention as powersources for small portable electronic devices. Rechargeable lithiumbatteries use an organic electrolyte solution, and thereby havedischarge voltages that are twice as high as batteries that use alkaliaqueous electrolyte solutions. Accordingly, rechargeable lithiumbatteries have high energy densities.

Rechargeable lithium batteries may be manufactured by injecting anelectrolyte into a battery cell including a positive electrode having apositive active material that can intercalate and deintercalate lithium,and a negative electrode having a negative active material that canintercalate and deintercalate lithium.

SUMMARY

One or more aspects of embodiments of the present disclosure provide arechargeable lithium battery having improved electrolyte impregnationproperties and thermal stability, as well as a high density activematerial electrode.

One or more aspects of embodiments of the present disclosure provide anegative active material for a rechargeable lithium battery thatincludes a carbon-based active material including highly crystallinenatural graphite and artificial graphite, wherein the carbon-basedactive material has a peak intensity ratio (P2/P4) of about 0.3 to about0.4, wherein P2 refers to the (101) peak intensity of a rhombohedralcrystal grain and P4 refers to the (101) peak intensity of a hexagonalcrystal grain, as measured by X-ray diffraction (XRD).

The carbon-based active material may have a peak intensity ratio (P2/P1)of about 0.43 to about 0.55, wherein P2 refers to the (101) peakintensity of a rhombohedral crystal grain and P1 refers to the (100)peak intensity of a hexagonal crystal grain.

The carbon-based active material may have a peak intensity ratio (P3/P4)of about 0.18 to about 0.26, where P3 refers to the (012) peak intensityof a rhombohedral crystal grain and P1 refers to the (101) peakintensity of a hexagonal crystal grain.

The carbon-based active material may have a Raman R value (I_(D)/I_(G))of 0.21≤I_(D)/I_(G)≤0.24.

The carbon-based active material may include the highly crystallinenatural graphite and the artificial graphite in a weight ratio of about10:90 to about 50:50.

The highly crystalline natural graphite may have a Raman R value(I_(D)/I_(G)) of 0.05≤I_(D)/I_(G)≤0.08.

The highly crystalline natural graphite may have a d₀₀₂ value of3.360<d₀₀₂ (Å)<3.365.

The negative active material for a rechargeable lithium battery mayfurther include a silicon-based active material.

The silicon-based active material may be present in an amount of about0.05 wt % to about 5 wt % based on the total amount of the negativeactive material in a rechargeable lithium battery.

One or more embodiments of the present disclosure are directed toward arechargeable lithium battery including a current collector; a negativeelectrode including the negative active material; a positive electrode;and an electrolyte.

The negative electrode may have an active material density of greaterthan or equal to about 1.7 g/cc.

Other embodiments are included in the following detailed description.

A rechargeable lithium battery cell having high active material density,improved electrolyte impregnation properties, and thermal stability maybe realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an cross-sectional view showing a rechargeable lithium batteryaccording to one embodiment.

FIG. 2 is a graph showing an XRD analysis of negative active materialaccording to one or more embodiments.

FIG. 3 is a graph showing the heat flow resulting from a temperaturechange.

FIG. 4 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Example 1.

FIG. 5 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Comparative Example 1.

FIG. 6 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Comparative Example 2.

FIG. 7 is a graph showing the XRD analysis of negative active materialaccording to Comparative Example 3.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described inmore detail with reference to the accompanying drawings, in whichexample embodiments of the disclosure are shown. As those skilled in theart would realize, the described embodiments may be modified in variousways, all without departing from the spirit or scope of the presentdisclosure.

In the drawings, the thickness of layers, films, panels, regions, etc.,may be exaggerated for clarity. Like reference numerals designate likeelements throughout the specification, and duplicative descriptionsthereof may not be provided. It will be understood that when an elementsuch as a layer, film, region, or substrate is referred to as being “on”another element, it can be directly on the other element or interveningelements may also be present. In contrast, when an element is referredto as being “directly on” another element, no intervening elements arepresent.

A negative active material for a rechargeable lithium battery accordingto one or more embodiments of the present disclosure includes acarbon-based active material including highly crystalline naturalgraphite and artificial graphite. As used herein, “highly crystalline”may refer to a state in which the larger portion of molecules in amaterial are arranged in grains of a regular repeating three-dimensionalstructure.

As for negative active materials, various carbon-based materials such asartificial graphite, natural graphite, hard carbon and the like havebeen used. Recently, non-carbon-based negative active materials such assilicon (Si) have been researched, in accordance with the need forstable and high-capacity batteries.

The carbon-based active material may have a peak intensity ratio (P2/P4)of about 0.3 to about 0.4, where P2 refers to the (101) peak intensityof a rhombohedral crystal grain and P4 refers to the (101) peakintensity of a hexagonal crystal grain when analyzed via X-raydiffraction.

When the ratio of the (101) peak intensity of a rhombohedral crystalgrain (P2) relative to the (101) peak intensity of a hexagonal crystalgrain (P4) is within this range, as measured from the XRD diffractionpattern collected with CuKα radiation, the hexagonal crystals and therhombohedral crystals are appropriately or suitably mixed, and may thussecure an impregnation path for an electrolyte solution even in anelectrode having a high active material density. Thus, a rechargeablelithium battery having excellent cycle-life characteristics may berealized.

The carbon-based active material may have a ratio (P2/P1) of about 0.43to about 0.55, where P2 refers to the (101) peak intensity of arhombohedral crystal grain and P1 refers to the (100) peak intensity ofa hexagonal crystal grain. The carbon-based active material may alsohave a ratio (P3/P4) of about 0.18 to about 0.26, where P3 refers to the(012) peak intensity of a rhombohedral crystal grain and P4 refers tothe (101) peak intensity of a hexagonal crystal grain.

In FIG. 2, the (101) peak intensity of the hexagonal crystal grain (P4)is marked as H(101), while the (101) peak intensity of the rhombohedralcrystal grain (P2) is marked as R(101). The (100) peak intensity of thehexagonal crystal grain (P1) is marked as H(100), while the (012) peakintensity of the rhombohedral crystal grain (P3) is marked as R(012).

The carbon-based active material may have a Raman R value (I_(D)/I_(G))of 0.21≤I_(D)/I_(G)≤0.24.

As used herein, I_(G) may indicate a peak arising from a crystallineregion (e.g., G-peak, a peak around 1580 cm⁻¹), while I_(D) may indicatea peak arising from an amorphous region (e.g., D-peak, a peak around1360 cm⁻¹), and the Raman R value may be defined as I_(D)/I_(G). Whenthe Raman R value is larger, the carbon-based active material may havelower crystallinity.

When the carbon-based active material has a Raman R value within theaforementioned range, a high density electrode plate may be realized,and the cycle-life characteristics of a battery may be improved. Whenthe Raman R value is less than about 0.21, the battery capacity may bedeteriorated or decreased due to deterioration or decrease of its chargeand discharge efficiency. However, when the Raman R value is greaterthan about 0.24, the swelling characteristics of a cell may becomeproblematic during extended cycling, and thus the cell may become thick.

The carbon-based active material may include highly crystalline naturalgraphite and artificial graphite in a weight ratio of about 10:90 toabout 50:50.

When the highly crystalline natural graphite is present in a ratio(e.g., an amount) of less than about 10 wt %, a stable high densitynegative electrode is difficult to manufacture, and the initial increasein battery capacity may be lost due to deterioration or decrease incharge/discharge efficiency during initial cycling. When the highlycrystalline natural graphite is present in a ratio (e.g., an amount) ofmore than about 50 wt %, the battery capacity may be increased, but theability to suppress battery swelling during long-term cycling may bedeteriorated or reduced.

In one or more embodiments, the highly crystalline natural graphite andthe artificial graphite may be mixed in (or to) a weight ratio of about10:90, about 20:80, and/or about 50:50, but embodiments of the presentdisclosure are not limited thereto.

The highly crystalline natural graphite included in the carbon-basedactive material according to one embodiment of the present disclosuremay have crystal characteristics that are different from those ofgeneral natural graphite, and may show crystal characteristics that aresimilar or close to those of artificial graphite.

In one or more embodiments, the highly crystalline natural graphite mayhave a Raman R value (I_(D)/I_(G)) of about 0.05≤I_(D)/I_(G)≤0.08, andd₀₀₂ may be about 3.360<d₀₀₂ (Å)<3.365.

As used herein, d₀₀₂ may indicate the distance between (002) planelayers in a XRD diffraction pattern obtained using CuKα radiation, andmay be used as an indicator of the crystallinity of the carbon-basedparticles along with the Raman R value.

According to one or more embodiments of the present disclosure, thehighly crystalline natural graphite included in a negative activematerial for a rechargeable lithium battery differs from general naturalgraphite. It is manufactured via additionally continuous (e.g.,substantially continuous) heat treatment at a high temperature andpressure, and thus may have appropriate or suitable crystallinity andpore characteristics. For example, general natural graphite ismanufactured through a pitch coating process, but spherical naturalgraphite is manufactured through a surface oxidation process, and maysecure crystallinity close to that of artificial graphite due to acapacity and efficiency increase according to removal of defected carbon(e.g., due to an increase in the capacity and efficiency of carbondefect removal) and an irreversibility decrease according to an —OHC═Odecrease by oxidization (e.g., an irreversible decrease in the presenceof —OH and C═O groups from oxidization). When the negative activematerial includes natural graphite having these crystal characteristics,the thermal stability of the battery and the impregnationcharacteristics of the electrolyte solution may be improved.

In some embodiments, the rechargeable lithium battery including thehighly crystalline natural graphite may have the beneficial features orcharacteristics of natural graphite in a high density active materialelectrode, and may thus provide high capacity as well as high energydensity due to the high density electrode.

Artificial graphite has excellent orientation characteristics. Whenmixed with the highly crystalline natural graphite in an appropriate orsuitable ratio, a rechargeable lithium battery having concurrently(e.g., simultaneously) improved cycle-life characteristics and batterycapacity may be realized.

In one or more embodiments, the artificial graphite of the carbon-basedactive material may have a peak intensity ratio of about 0.62 to about0.64, as calculated for the (101) peak intensity of a rhombohedralcrystal grain relative to the (100) peak intensity of a hexagonalcrystal grain.

The negative active material for a rechargeable lithium battery mayfurther include a silicon-based active material. When the negativeactive material further includes a silicon-based active material, a highcapacity lithium battery may be realized.

The silicon-based active material may be present in an amount of about0.05 wt % to about 5 wt % based on the total amount of the negativeactive material for a rechargeable lithium battery. When thesilicon-based active material is included in an amount of less thanabout 0.05 wt %, the improvement in battery capacity may be deterioratedor diminished; when the silicon-based active material is included in anamount greater than about 5 wt %, the effect of suppressing batteryswelling may be deteriorated or diminished due to electrode expansioncaused by the increased amount of silicon.

In some embodiments, the silicon-based active material may be includedin an amount of about 0.05 wt % to about 3.5 wt % and in someembodiments, about 0.05 wt % to about 1 wt %, but embodiments of thepresent disclosure are not limited thereto and the amounts may beadjusted, depending on desired capacity.

The silicon-based active material may be a material selected from Si,SiO_(x) (0<x<2), a Si—Z alloy (wherein Z may be selected from an alkalimetal, an alkaline-earth metal, a Group 13 element, a Group 14 elementexcluding Si, a transition metal, a rare earth element, and/or acombination thereof), and a combination thereof. As used herein, theterms “combination”, “combination thereof”, and “combinations thereof”may refer to a chemical combination (e.g., an alloy or chemicalcompound), a mixture, or a laminated structure of components.

Hereinafter, a rechargeable lithium battery including the negativeactive material will be described with reference to FIG. 1.

FIG. 1 is a cross-sectional view showing a rechargeable lithium batteryaccording to an embodiment of the present disclosure.

Referring to FIG. 1, the rechargeable lithium battery 1 is a prismaticbattery that includes an electrode assembly including a positiveelectrode 2, a negative electrode 3, and a separator 4 between thepositive electrode 2 and the negative electrode 3 in a battery case 5,an electrolyte solution injected through the upper part of the batterycase, and a cap plate 6 for sealing the battery. The rechargeablelithium battery is not limited to a prismatic shape, but may, forexample, have a cylindrical, coin-type (e.g., coin), or pouch shape aslong as the rechargeable lithium battery including the negative activematerial for a rechargeable lithium battery can be suitably operated.

In one or more embodiments, a rechargeable lithium battery includes acurrent collector; a negative electrode including a negative activematerial; a positive electrode; and an electrolyte.

The negative active material may be the same as described above.

The negative electrode may have an active material density greater thanor equal to about 1.7 g/cc.

The negative electrode may have an active material density greater thanor equal to about 1.7 g/cc, and in some embodiments about 1.7 g/cc toabout 1.8 g/cc, and in some embodiments about 1.7 g/cc to about 1.72g/cc. Since the energy density of the negative electrode may beincreased due to the high density of the active material, the capacitycharacteristics of the battery may be improved.

The active material density of an electrode may be obtained by dividingthe total mass of components excluding the current collector (e.g., theactive material, conductive material, binder, and/or the like) by thetotal volume, and is reported in units of g/cc. In general, althoughexcellent battery capacity may be obtained when the electrode has ahigher active material density, the cycle-life characteristics of theelectrode may be deteriorated.

However, in one or more example embodiments, the negative activematerial for a rechargeable lithium battery includes a mixture of theartificial graphite having excellent orientation and the highlycrystalline natural graphite, and may exhibit excellent battery capacitywithout a deterioration in cycle-life.

In other words, the negative active material for a rechargeable lithiumbattery may include a novel carbon-based active material includinghighly crystalline natural graphite and artificial graphite, and mayexhibit improved or increased thermal stability and cycle-lifecharacteristics as well as facilitate the use of high density activematerial electrodes.

The current collector material may be selected from a copper foil, anickel foil, a stainless steel foil, a titanium foil, a nickel foam, acopper foam, a polymer substrate coated with a conductive metal, and acombination thereof, but embodiments of the present disclosure are notlimited thereto.

The negative electrode may include a layer of negative active materialon the current collector, and the layer of negative active material mayfurther include a binder, a conductive material and/or a thickener inaddition to the negative active material.

The negative active material may be the same as described above.

The binder may improve the binding properties of the negative activematerial particles with one another and with a current collector.Non-limiting examples of the binder material may include polyvinylalcohol, carboxymethyl cellulose, hydroxypropyl cellulose,polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, anethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride (PVdF), polyethylene(PE), polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like.

The conductive material may improve the electrical conductivity of theelectrode. Any electrically conductive material may be used as aconductive material, unless it causes a chemical change (e.g., anundesirable change in the fabricated battery). Non-limiting examples ofthe conductive material may include a carbon-based material (e.g.,natural graphite, artificial graphite, carbon black, acetylene black,Ketjenblack, carbon fiber, and/or the like); a metal-based material(e.g., copper (Cu), nickel (Ni), aluminum (Al), silver (Ag), and/or thelike in the form of metal powder, metal fiber, and/or the like); aconductive polymer (e.g., a polyphenylene derivative and/or the like);and/or a mixture thereof.

The thickener may be an additive for increasing the viscosity of thenegative active material slurry, and may be, for example, carboxylmethyl cellulose (CMC), but embodiments of the present disclosure arenot limited thereto.

The positive electrode may include a current collector and a positiveactive material layer formed on the current collector. The positiveactive material may include lithiated intercalation compounds thatreversibly intercalate and deintercalate lithium ions. In one or moreembodiments, at least one composite oxide of lithium and a metal ofcobalt, manganese, nickel, or a combination thereof may be used.Non-limiting examples thereof may include a compound represented by oneof the following chemical formulae:

Li_(a)A_(1-b)X_(b)D₂ (0.90≤a≤1.8, 0≤b≤0.5);Li_(a)A_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(1-b)X_(b)O_(2-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(2-b)X_(b)O_(4-c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)Ni_(1-b-c)CO_(b)X_(c)D_(a) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0≤a≤2);Li_(a)Ni_(1-b-c)CO_(b)X_(c)O_(2-a)T_(a) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05,0≤a≤2); Li_(a)Ni_(1-b-c)CO_(b)X_(c)O_(2-a)T₂ (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0≤a≤2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)D_(a) (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0≤a≤2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-a)T_(a) (0.90≤a≤1.8,0≤b≤0.5, 0≤c≤0.05, 0≤a≤2); Li_(a)Ni_(1-b-c)Mn_(b)X_(c)O_(2-a)T₂(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05, 0≤a≤2); Li_(a)Ni_(b)E_(c)G_(d)O₂(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1);Li_(a)Ni_(b)CO_(c)Mn_(d)G_(e)O₂ (0.90≤a≤1.8, 0≤b≤10.9, 0≤c≤0.5, 0≤d≤0.5,0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)CoG_(b)O₂(0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1-b)G_(b)O₂ (0.90≤a≤1.8,0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄ (0.90≤a≤1.8, 0.001≤b≤0.1);Li_(a)Mn_(1-g)G_(g)PO₄ (0.90≤a≤1.8, 0≤g≤0.5); QO₂; QS₂; LiQS₂; V₂O₅;LiV₂O₅; LiZO₂; LiNiVO₄; Li_((3-f))J₂(PO₄)₃ (0≤f≤2); Li_((3-f))Fe₂(PO₄)₃(0≤f≤2); LiFePO₄

In Chemical Formulae, A may be selected from nickel (Ni), cobalt (Co),manganese (Mn), and a combination thereof; X may be selected fromaluminum (Al), Ni, Co, Mn, chromium (Cr), iron (Fe), magnesium (Mg),strontium (Sr), vanadium (V), a rare earth element, and a combinationthereof; D may be selected from oxygen (O), fluorine (F), sulfur (S),phosphorus (P), and a combination thereof; E may be selected from Co,Mn, and a combination thereof; T may be selected from F, S, P, and acombination thereof; G may be selected from Al, Cr, Mn, Fe, Mg,lanthanum (La), cerium (Ce), Sr, V, and a combination thereof; Q may beselected from titanium (Ti), molybdenum (Mo), Mn, and a combinationthereof; Z may be selected from Cr, V, Fe, Sc, yttrium (Y), and acombination thereof; and J may be selected from V, Cr, Mn, Co, Ni,copper (Cu), and combination thereof.

This compound may have a coating layer on its surface or may be mixedwith another compound having the coating layer. This coating layer mayinclude at least one coating element compound selected from an oxide ofa coating element, a hydroxide of the coating element, an oxyhydroxideof the coating element, an oxycarbonate of the coating element, and ahydroxycarbonate of the coating element. The compound for forming thecoating layer may be amorphous or crystalline. The coating elementincluded in the coating layer may be selected from Mg, Al, Co, potassium(K), sodium (Na), calcium (Ca), silicon (Si), Ti, V, tin (Sn), germanium(Ge), gallium (Ga), boron (B), astatine (As), zirconium (Zr), and amixture thereof. The coating layer may be formed using any suitablemethod having no negative influence on the positive active material(e.g., spray coating, dipping, and/or the like). Such methods will beeasily understood by those who have knowledge in the related art and arenot illustrated in more detail.

The positive active material may be included in an amount of about 80 toabout 99 wt % based on the total amount of material in the positiveactive material layer. The positive active material layer may alsoinclude a binder and a conductive material. Herein, the binder and theconductive material may be respectively used in amounts of about 1 toabout 5 wt % based on the total amount of the positive active materiallayer.

The binder may improve the binding properties of the positive activematerial particles with one another and with a current collector.Non-limiting examples thereof may include polyvinyl alcohol,carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, anethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, and the like.

The conductive material may improve the conductivity of the electrode.Any electrically conductive material may be used as a conductivematerial unless it causes a chemical change (e.g., an undesirable changein the fabricated battery). Non-limiting examples of the conductivematerial may include one or more of natural graphite, artificialgraphite, carbon black (e.g., acetylene black, and/or Ketjenblack), acarbon fiber, a metal powder, or a metal fiber of copper, nickel,aluminum, silver, and the like, and a polyphenylene derivative and thelike.

The electrolyte may include a non-aqueous organic solvent and a lithiumsalt.

The non-aqueous organic solvent may serve as a medium for transmittingthe ions taking part in the electrochemical reaction of a battery.

The organic solvent may include one or more solvents selected from acarbonate-based solvent, an ester-based solvent, an ether-based solvent,a ketone-based solvent, an alcohol-based solvent, and an aproticsolvent. The carbonate based solvent may include, for example dimethylcarbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC),methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethylcarbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC),butylene carbonate (BC), and the like, and the ester-based solvent mayinclude, for example, methyl acetate, ethyl acetate, n-propyl acetate,1,1-dimethylethyl acetate, methylpropionate, ethylpropionate,γ-butyrolactone, decanolide, valerolactone, mevalonolactone,caprolactone, and the like. The ester-based solvent may include, forexample, dibutyl ether, tetraglyme, diglyme, dimethoxyethane,2-methyltetrahydrofuran, tetrahydrofuran, and the like, and theketone-based solvent may include, for example, cyclohexanone and thelike. The alcohol-based solvent may include, for example, ethanol,isopropyl alcohol, and the like, and the aprotic solvent may includenitriles such as R—CN (wherein R is a C₂ to C₂₀ linear, branched, orcyclic hydrocarbon group, and may include a double bond, an aromaticring, or an ether bond), amides such as dimethylformamide, dioxolanessuch as 1,3-dioxolane, sulfolanes, and the like.

The non-aqueous organic solvent may be used singularly or in a mixture.When the organic solvent is used in a mixture, the mixture ratio may bechosen according to the desired or suitable battery performance.

The carbonate-based solvent may be prepared by mixing a cyclic carbonateand a linear carbonate. The cyclic carbonate and the linear carbonatemay be mixed together in (or to) a volume ratio of about 1:1 to about1:9. When the volume ratio is within this range, the performance of theelectrolyte may be improved.

The non-aqueous organic electrolyte may be further prepared by mixing acarbonate-based solvent with an aromatic hydrocarbon-based solvent. Thecarbonate-based solvents and the aromatic hydrocarbon-based solvents maybe mixed together in (or to) a volume ratio of about 1:1 to about 30:1.

The aromatic hydrocarbon-based organic solvent may be an aromatichydrocarbon-based compound represented by Chemical Formula 1:

In Chemical Formula 1, R₁ to R₆ may each independently be selected fromhydrogen, a halogen, a C₁ to C₁₀ alkyl group, a C₁ to C₁₀ haloalkylgroup, or a combination thereof.

The aromatic hydrocarbon-based organic solvent may include benzene,fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene,1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene,chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene,1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene,iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene,1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene,1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene,1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene,1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene,1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene,1,2,3-triiodotoluene, 1,2,4-triiodotoluene, xylene, or a combinationthereof.

The non-aqueous electrolyte may further include vinylene carbonate or anethylene carbonate-based compound represented by Chemical Formula 2 inorder to improve battery cycle-life:

In Chemical Formula 2, R₇ and R₈ may each independently be selected fromhydrogen, a halogen, a cyano group (CN), a nitro group (NO₂) or a C₁ toC₅ fluoroalkyl group, provided that at least one of the R₇ and R₈ is ahalogen, a cyano group (CN), a nitro group (NO₂) or a C₁ to C₅fluoroalkyl group.

Non-limiting examples of the ethylene carbonate-based compound mayinclude difluoroethylene carbonate, chloroethylene carbonate,dichloroethylene carbonate, bromoethylene carbonate, dibromoethylenecarbonate, nitroethylene carbonate, cyanoethylene carbonate,fluoroethylene carbonate, and the like. The amount of the vinylenecarbonate or the ethylene carbonate-based compound used to improve cyclelife may be adjusted within an appropriate or suitable range.

The lithium salt may be dissolved in an organic solvent, and may supplylithium ions in a battery. The lithium salt may facilitate basicoperation of the rechargeable lithium battery, and may improve lithiumion transport between the positive and negative electrodes therein.Non-limiting examples of the lithium salt may include at least onesupporting salt of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiC₄F₉SO₃, LiClO₄,LiAlO₂, LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂), wherein x andy are natural numbers, LiCl, LiI, LiB(C₂O₄)₂ (e.g., lithium bis(oxalato)borate; LiBOB), and/or a combination thereof. The lithium salt may beused in a concentration of about 0.1 M to about 2.0 M. When the lithiumsalt is included at a concentration within the above range, theelectrolyte may have excellent lithium ion mobility and performance dueto optimal or suitable electrolyte conductivity and viscosity.

The separator 113 may include any material commonly used in lithiumbatteries, as long as it is able to suitably separate the negativeelectrode 112 from the positive electrode 114 and provide a transportingpassage for lithium ions. In other words, the separator may have a lowion transport resistance and excellent electrolyte solution impregnationcharacteristics. The separator material may be selected from, forexample, glass fiber, polyester, TEFLON® (e.g., polytetrafluoroethylene,PTFE), polyethylene, polypropylene, and/or a combination thereof. It maybe formed as a non-woven fabric or a woven fabric. For example, apolyolefin-based polymer separator such as polyethylene, polypropylene,or the like is commonly used in lithium ion batteries. A coatedseparator including a ceramic or polymer component may be used toincrease the heat resistance and/or mechanical strength of theseparator. The separator may have a mono-layered or multi-layeredstructure.

Hereinafter, a method of manufacturing one or more example embodimentsof the negative active material will be illustrated in more detail.

First, natural graphite is processed into a sphere, preparing aspherically-shaped natural graphite.

The spherically-shaped natural graphite is heat-treated andpressure-treated, preparing highly crystalline natural graphite.

The heat treatment may be performed at about 600° C. to about 700° C.for about 8 to about 16 hours, and in some embodiments, at about 620° C.to about 650° C. for about 8 to about 12 hours. The method ofmanufacturing the highly crystalline natural graphite according to anexample embodiment does not include a pitch coating process and thus mayprevent or reduce graphitization under high pressure conditions. Inother words, the method maintains high crystallinity and thus may beused to realize excellent electrolyte solution impregnation, therebyimprove the cycle-life characteristics.

The highly crystalline natural graphite may be pressure-treated afterthe heat treatment.

The pressure treatment may be performed at a pressure of about 20 toabout 120 MPa and in some embodiments, at about 50 to about 100 MPa.Before the pressure treatment, a plurality of pores may be formed insidethe highly crystalline natural graphite, through which an electrolytesolution permeates, causing a continuous irreversible reaction. Thisirreversible reaction may degrade the cycle-life characteristics of thebattery due to a decrease in capacity and a thickness increase (e.g., anincrease in the thickness of the battery), and thus, embodiments of thepresent disclosure improve the characteristics of the battery bydecreasing the pores (e.g., by decreasing the size and/or amount of thepores).

In other words, the highly crystalline natural graphite included in anegative active material for a rechargeable lithium battery according toan example embodiment may be formed through continuous (e.g.,substantially continuous) heat and pressure treatment and thus may haveappropriate or suitable crystallinity and pore characteristics.

The highly crystalline natural graphite may have a Raman R value(I_(D)/I_(G)) of about 0.05≤I_(D)/I_(G)≤about 0.08.

The highly crystalline natural graphite may have a d₀₀₂ of about3.360<d₀₀₂ (Δ)<3.365.

The highly crystalline natural graphite may be mixed with artificialgraphite.

The mixing of the highly crystalline natural graphite with theartificial graphite may be performed via a mechanical mixing method. Forexample, the mixing may be performed utilizing a method selected fromball milling, mechanofusion milling, shaker milling, planetary milling,attritor milling, disk milling, shape milling, Nauta milling, Nobiltamilling, and/or a combination thereof, but embodiments of the presentdisclosure are not limited thereto.

The highly crystalline natural graphite and the artificial graphite maybe mixed in (or to) a weight ratio of about 10:90 to about 50:50 and insome embodiments, about 20:80 to about 30:70, but embodiments of thepresent disclosure are not limited thereto.

A silicon-based active material may be further added to the mixture ofthe highly crystalline natural graphite with the artificial graphite.

The silicon-based active material may be used in an amount of about 0.05wt % to about 5 wt %, in some embodiments, about 0.05 to about 3.5 wt %,and in some embodiments, about 0.05 to about 1 wt %, but embodiments ofthe present disclosure are not limited thereto and the amount ofsilicon-based active material may be adjusted according to the desiredor suitable battery capacity.

Hereinafter, aspects of the present disclosure will be illustrated inmore detail with reference to example embodiments. However, embodimentsof the present disclosure are not limited thereto.

Manufacture of Negative Electrode Manufacture of Highly CrystallineNatural Graphite

Natural graphite (Raman R value: 0.05, average particle diameter (D50):17 μm) was treated at 620° C. for 10 hours in a rotary kiln,manufacturing highly crystalline natural graphite.

Example 1

The highly crystalline natural graphite was mixed with artificialgraphite in (or to) a weight ratio of 10:90 to obtain a carbon-basedactive material, and 99 wt % of the carbon-based active material wasmixed with 1 wt % of SiOx (0<x<2) to prepare a negative active material,

97.5 wt % of the negative active material, 1.5 wt % of styrene-butadienerubber as a binder, and 1 wt % of carboxymethyl cellulose as a thickenerwere mixed and then dispersed into water to prepare a negative activematerial slurry. Subsequently, the negative active material slurry wascoated on a copper foil via a slot die method to form a negative activematerial layer, and the negative active material layer was dried at 80°C.

Subsequently, the copper foil coated with the negative active materiallayer was pushed into a compression roll and pressed therein, and thecompression roll was adjusted to have a compression density of about1.75 g/cc.

The compressed negative active material layer was secondarily dried atabout 140° C. under vacuum, forming a negative electrode.

Example 2

A negative electrode was manufactured according to substantially thesame method as in Example 1, except for mixing the highly crystallinenatural graphite and the artificial graphite in (or to) a weight ratioof 20:80.

Example 3

A negative electrode was manufactured according to substantially thesame method as in Example 1, except for mixing the highly crystallinenatural graphite and the artificial graphite in (or to) a weight ratioof 50:50.

Comparative Example 1

A negative electrode was manufactured according to substantially thesame method as in Example 1, except for mixing the highly crystallinenatural graphite and the artificial graphite in (or to) a weight ratioof 80:20.

Comparative Example 2

A negative electrode was manufactured according to substantially thesame method as in Example 1, except for preparing a negative activematerial by mixing 99 wt % of a carbon-based active material comprisingthe highly crystalline natural graphite and 1 wt % of SiO_(x) (0<x<2).

Comparative Example 3

A negative electrode was manufactured according to substantially thesame method as in Example 1, except for preparing a negative activematerial by mixing 99 wt % of a carbon-based active material comprisingthe artificial graphite and 1 wt % of SiO_(x)(0<x<2).

Evaluation 1: X-Ray Diffraction (XRD) of Highly Crystalline NaturalGraphite

The XRD patterns of the negative active materials according to Examples1 to 3 are provided in FIG. 2. The XRD patterns of the negative activematerials according to Comparative Example 3 are provided in FIG. 7.

FIG. 2 shows that as the ratio amount of highly crystalline naturalgraphite increased, the (101) peak intensity (P2) corresponding to arhombohedral crystal grain was increased.

The increase in (101) peak intensity (P2) of the rhombohedral crystalgrain may indicate that battery capacity can be increased by maintaininghigh active material density in the electrode. As shown in FIG. 7,Comparative Example 3, which does not include the highly crystallinenatural graphite, had a (P2/P1) ratio of about 0.62 to 0.64, where P2 isthe (101) peak intensity of a rhombohedral crystal grain and P1 is the(100) peak intensity of a hexagonal crystal grain. Comparative Example 3is thus expected to have an insufficient or lowered capacity improvementeffect during initial charge/discharge due to deterioration ofefficiency.

Evaluation 2: Impregnation Degree of Electrolyte Solution

The degree to which an electrolyte solution was impregnated into thenegative electrodes of Example 1 and Comparative Examples 1 and 2 wasevaluated.

The electrolyte solution was prepared using ethylene carbonate(EC)/ethylmethyl carbonate (EMC)/dimethyl carbonate (DMC) mixed at (orto) a ratio of 3/3/4 (v/v/v) and 1.15 M LiPF₆.

The impregnation was performed in the following two ways.

The first method was performed by dropping 20 mg of the electrolytesolution on the negative electrodes of Example 1 and ComparativeExamples 1 and 2 until the electrolyte solution was completely soakedtherein.

The results are provided in Table 1:

TABLE 1 Impregnation time Impregnation speed (second) (ref = 100%)Example 1 270 1.00 Comparative Example 1 360 1.33 Comparative Example 2390 1.44

Referring to Table 1, the negative electrode of Example 1 showed ashorter impregnation time and a higher impregnation speed compared withthe negative electrodes of Comparative Examples 1 and 2.

The second method was performed by cutting the negative electrodes ofExample 1 and Comparative Examples 1 and 2 to a width of 2 cm, placingthem vertically in the electrolyte solution to a depth of 2 mm for 3minutes, and then measuring the weight change of the negative electrodesto measure the weight of the electrolyte solution adsorbed therein. Theevaluation was performed using a Sigma 700 (made by Attention).

The results are provided in Table 2:

TABLE 2 Mass increase (mg) Impregnation speed (180 sec) (ref = 100%)Example 1 9.03 144 Comparative Example 1 8.19 131 Comparative Example 26.98 111

Referring to Table 2, the negative electrode of Example 1 showed a highmass increase compared with the negative electrodes of ComparativeExamples 1 and 2. Accordingly, the negative electrode of Example 1 wasmore impregnated by the electrolyte solution than the negativeelectrodes of Comparative Examples 1 and 2.

The two experimental results show that a negative electrode using anactive material slurry that includes highly crystalline natural graphitemay have a plurality of pores in the negative active material layer thatthus provide an impregnation path for an electrolyte solution andresultantly increase the impregnation speed of the electrolyte solution.

Evaluation 3: Thermal Stability

The heat flows of highly crystalline natural graphite (New NG) accordingto Example 1 and pitch-coated natural graphite (NG) were measured bydifferential scanning calorimetry (DSC) between 100-200° C. at a speedof 10° C./min, and the results are provided in FIG. 3.

FIG. 3 is a graph showing the heat flow of the samples according to thechange in temperature.

FIG. 3 shows that highly crystalline natural graphite (New NG), asdescribed in the present disclosure, showed initial characteristics ofthermal stability.

Evaluation 4: Cycle-Life Characteristics

A rechargeable lithium battery cell manufactured via a method thatshould be readily apparent to those of ordinary skill in the art (e.g.,using a PVdF-based binder and a PE separator coated with a polymer onboth sides) was charged using a constant current-constant voltage(CC/CV) method of 0.7 C to 4.35 V with an ending current of 90 mA,discharged to 3.0 V at a constant current of 0.5 C, and then evaluatedregarding cycle-life characteristics. The results are provided in FIGS.4 to 6.

FIG. 4 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Example 1.

FIG. 5 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Comparative Example 1.

FIG. 6 is a graph showing the cycle-life characteristics of therechargeable lithium battery cell of Comparative Example 2.

Referring to FIGS. 4 to 6, the rechargeable lithium battery cell ofExample 1 maintained about 85 to 90% of its capacity at the 400th cycleand thus showed excellent cycle-life characteristics compared with therechargeable lithium battery cells of Comparative Examples 1 and 2.

As used herein, expressions such as “at least one of” and “one of”, whenpreceding a list of elements, modify the entire list of elements and donot modify the individual elements of the list. Further, the use of“may” when describing embodiments of the present disclosure refers to“one or more embodiments of the present disclosure”.

In addition, as used herein, the terms “use”, “using”, and “used” may beconsidered synonymous with the terms “utilize”, “utilizing”, and“utilized”, respectively.

As used herein, the terms “substantially”, “about”, and similar termsare used as terms of approximation and not as terms of degree, and areintended to account for the inherent deviations in measured orcalculated values that would be recognized by those of ordinary skill inthe art.

Also, any numerical range recited herein is intended to include allsub-ranges of the same numerical precision subsumed within the recitedrange. For example, a range of “1.0 to 10.0” is intended to include allsubranges between (and including) the recited minimum value of 1.0 andthe recited maximum value of 10.0, that is, having a minimum value equalto or greater than 1.0 and a maximum value equal to or less than 10.0,such as, for example, 2.4 to 7.6. Any maximum numerical limitationrecited herein is intended to include all lower numerical limitationssubsumed therein and any minimum numerical limitation recited in thisspecification is intended to include all higher numerical limitationssubsumed therein. Accordingly, Applicant reserves the right to amendthis specification, including the claims, to expressly recite anysub-range subsumed within the ranges expressly recited herein.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the disclosure is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims and equivalents thereof.

DESCRIPTION OF SOME OF THE SYMBOLS

-   1: rechargeable lithium battery-   2: positive electrode-   3: negative electrode-   4: separator-   5: battery case-   6: cap plate

What is claimed is:
 1. A negative active material for a rechargeablelithium battery, comprising: a carbon-based active material includinghighly crystalline natural graphite and artificial graphite, wherein thecarbon-based active material has a peak intensity ratio (P2/P4) of 0.3to 0.4, wherein P2 refers to a (101) peak intensity of a rhombohedralcrystal grain and P4 refers to a (101) peak intensity of a hexagonalcrystal grain, as measured by X-ray diffraction, and wherein the highlycrystalline natural graphite is heat-treated, surface oxidized sphericalnatural graphite without a pitch coating.
 2. The negative activematerial of claim 1, wherein the carbon-based active material has a peakintensity ratio (P3/P4) of about 0.18 to about 0.26, wherein P3 refersto a (012) peak intensity of a rhombohedral crystal grain, as measuredby X-ray diffraction.
 3. The negative active material of claim 1,wherein the highly crystalline natural graphite has a Raman R value(I_(D)/I_(G)) of 0.05≤I_(D)/I_(G)≤0.08.
 4. The negative active materialfor a rechargeable lithium battery of claim 1, wherein the negativeactive material further comprises a silicon-based active material. 5.The negative active material for a rechargeable lithium battery of claim4, wherein the silicon-based active material is present in an amount ofabout 0.05 wt % to about 5 wt % based on the total amount of thenegative active material for a rechargeable lithium battery.
 6. Arechargeable lithium battery comprising: a current collector; a negativeelectrode including the negative active material of claim 1; a positiveelectrode; and an electrolyte.
 7. The rechargeable lithium battery ofclaim 6, wherein the negative electrode has an active material layerdensity of greater than or equal to about 1.7 g/cc.